Size reduction of semiconductor devices has enabled continued improvement in speed, performance, density, and cost per unit function of integrated circuits and systems. As process nodes continue to shrink, critical dimensions (“CD”) of the semiconductor devices have approached and even surpassed the theoretical limits of photolithography equipment. An approach to achieve finer resolution with photolithographic equipment is to use immersion photolithography. In immersion photolithography, a liquid such as purified water is interposed between the semiconductor substrate having photosensitive resist on it, and the final lens or patterning mask. By using a medium instead of air between the lens and the photoresist to be exposed, and in particular a medium having a refractive index greater than one, the achievable resolution may be increased. Immersion photolithography may increase resolution by as much as 30 percent over ‘dry’ processes.
As the feature sizes are reduced and the minimum feature sizes drop to 45, 32, 28 and lower nanometers, the sensitivity of the photolithographic process to unwanted particles increases. Particles have been observed in immersion photolithographic processes. These particles may become mobile in the immersion medium and can cause image defocusing during the photoresist exposure process, resulting in blurred lines in the exposed photoresist and corresponding defects in the patterned devices. Lowered yield and increased cost for finished devices may therefore result.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily drawn to scale.